The present invention generally relates to power supplies for vehicle accessories, and more particularly pertains to a power supply for use in supplying power to an electrochromic mirror.
Automotive power systems are migrating from 12 V in cars and 24 V in some trucks towards higher voltages such as 42 V to reduce wiring weight and provide more electrical power for systems such as electric power steering and heat pumps. Electrochromic mirrors are driven with low voltages at relatively high currents. When using a linear series regulated power supply in a 12 V system, the power dissipation in the mirror can be in the range of 4 to 5 W (350 mA load). Using the same design approach with a 42 V supply would result in a 15 W power dissipation. A power dissipation of 5 W is in itself fairly difficult to handle in a small object such as a rearview mirror. A 15 W power dissipation presents a serious problem. Some mirrors, such as those that include displays or even dual displays (see, for example, commonly assigned U.S. Pat. No. 6,346,698), can draw in excess of 500 mA and present an even larger and greater challenge with respect to thermal dissipation.
While voltage converters are generally known that could conceivably be utilized to convert a 42 V supply to lower, more practical voltages, such switched power supplies generate significant levels of electromagnetic interference (EMI). The rearview mirror is in a particularly critical location due to its proximity to the vehicle""s radio antenna and the lack of a ground plane. To reduce cost, rearview mirrors are typically placed in plastic housings, which provide little EMI shielding. Thus, using such a switched power supply in a rearview mirror would result in EMI levels that greatly exceed the maximum levels accepted by manufacturers.
It is an aspect of the present invention to provide a rearview mirror assembly that can receive power from a power supply source of the vehicle having a voltage in excess of 24 V while exhibiting EMI levels within acceptable limits for a rearview mirror. To achieve these and other aspects and advantages, a rearview mirror assembly of the present invention comprises a housing adapted to be mounted in the vehicle, a mirror disposed in the housing, at least one electrically powered device disposed in the housing, and a power supply for receiving power from a vehicle power source having a voltage in excess of about 24 V and supplying power at a voltage of about 5 V or less to the electrically powered device. The electrically powered device and the power supply exhibit an electromagnetic interference level less than about 41 dBxcexcV/m for emissions in the frequency range from about 0.4 MHz to about 20 MHz. The mirror may be an electrochromic mirror, while the electrically powered device may be a control circuit for the electrochromic mirror. The electrically powered device may also, or alternatively, be one or more of the following: a display device, map lights, outside electrochromic rearview mirrors, an electronic compass circuit, a GPS receiver, a trainable transceiver, and a cellular telephone transceiver.